1. Field of the Invention
The present invention relates to an exhaust apparatus and a vacuum pumping unit including the exhaust apparatus, which are specifically adapted for discharging a gas in a vacuum vessel to produce an ultrahigh vacuum in the semiconductor process or the like.
2. Prior Art
FIG. 12 conceptionally illustrates the prior art vacuum equipment including a high vacuum pump, wherein a vacuum chamber 1 is connected to a vacuum pump 2 through an exhaust pipe 3. The vacuum pump 2, which comprises, for example, a turbo molecular pump, on oil diffusion pump, an ion pump and the like and, exhausts only the gas molecules which fly into the exhaust pipe 3 from the vacuum chamber 1.
However, in the above-constructed vacuum equipment, when a turbo molecular pump is used to exhaust a gas with a low compression ratio such as hydrogen, helium or the like, gas molecules may diffuse back to a high vacuum side i.e. to the vacuum chamber 1 and, thus causes a decrease in the vacuum level.
In the case of an oil diffusion pump, the gas molecules, which were once exhausted by the pump, may flow back into the vacuum chamber and further, the vapor of pumping oil heated also diffuses back. Thus, the vacuum level decreases.
In the case of the ion pump, gas molecules absorbed into a titanium wall of the pump are released and flow back into the vacuum chamber, thus reducing the vacuum level.
In the prior art, no effective means were available against the back diffusion of gas molecules with a low compression ratio or released gas molecules from the vacuum pump. Nevertheless, oil vapor from the oil diffusion pump may be prevented only by providing a cold trap with liquid nitrogen. However, complete prevention for any counterflow has been very difficult.